Free Fatty Acid Metabolism
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Direct answer
Between meals, adipose triglyceride is dismantled into non-esterified (free) fatty acids that enter plasma bound to albumin — up to roughly ten binding sites per molecule — and travel to heart, muscle and liver for beta-oxidation. Lipolysis is initiated by adipose triglyceride lipase and driven by hormone-sensitive lipase, which catecholamines activate through a cAMP-dependent phosphorylation cascade and insulin suppresses by rephosphorylation via phosphodiesterase 3B. Uptake needs CD36 and the fatty acid transport proteins; mitochondrial entry requires the carnitine shuttle, whose carnitine palmitoyltransferase-1 is inhibited by malonyl-CoA — the switch that diverts fatty acids to ketogenesis during fasting. Complete palmitate oxidation yields 8 acetyl-CoA and, by current estimates, about 106 ATP.
What you must remember
- Control architecture: catecholamines and glucagon activate hormone-sensitive lipase (cyclic AMP, protein kinase A, phosphorylation — the classic amplifying cascade); insulin shuts it off and simultaneously drives re-esterification through glycerol-3-phosphate from glucose uptake.
- Albumin carriage: FFAs are the most abundant lipid fuel in plasma yet the least abundant by pool size because turnover is rapid (half-life of a few minutes); glycerol released with them cannot be re-esterified in adipose tissue, which lacks glycerol kinase.
- Carnitine shuttle numbers: CPT-1 on the outer membrane is rate-limiting for mitochondrial import and is inhibited by malonyl-CoA (abundant in the fed state when acetyl-CoA carboxylase is active); CPT-2 reloads acyl-CoA inside.
- Yield arithmetic: palmitate (C16) undergoes 7 spirals producing 7 FADH2, 7 NADH and 8 acetyl-CoA — approximately 106 ATP after costs, or 129 by older counts; quote "about 106, per current estimates" in vivas to be safe.
- Energy densities: heart derives most resting ATP from fatty acid oxidation; brain cannot use FFAs (albumin-bound, no blood-brain transit) and switches to ketone bodies instead.
- Medium-chain acyl-CoA dehydrogenase deficiency is the commonest beta-oxidation defect — hypoketotic hypoglycaemia with dicarboxylic aciduria, flagged by octanoylcarnitine (C8) on newborn screening.
- Clinical states of lipolysis: uncontrolled diabetes and DKA (FFAs flood the liver, ketones pour out), stress and sepsis (myocardial FFA excess is arrhythmogenic), and rapid weight loss or glucocorticoid excess.
Fasting physiology walked through, hour by hour
After an overnight fast, insulin falls and glucagon and catecholamines rise; within minutes, hormone-sensitive lipase is phosphorylated and adipocyte triglyceride releases FFA and glycerol. The FFA-albumin complex reaches the liver, where two destinies compete: beta-oxidation to acetyl-CoA fuelling the Krebs cycle, or acetyl-CoA diverted — because oxaloacetate is being consumed for gluconeogenesis — to ketone bodies. By 3 days of fasting, hepatic ketogenesis supplies a growing share of the brain's fuel, sparing protein. Meanwhile glycerol from lipolysis enters gluconeogenesis as a genuine net glucose precursor (the fatty acid chain cannot yield glucose in humans, a two-mark certainty). Exercise rewires the same pathway: catecholamine-driven lipolysis, AMPK-mediated fatty acid entry and oxidation in muscle, with trained muscle oxidising fat at higher work rates. Now appreciate the diabetic failure mode: in DKA the identical machinery runs without brake — profound insulin deficiency means lipolysis is unrestrained, hepatic ketogenesis is massive, and the hypoketotic state of MCAD deficiency is its exact biochemical mirror image.
Where students slip
Two sentences fail vivas every year. "Fatty acids make glucose" — they do not in humans; even-chain acetyl-CoA has no net path to oxaloacetate, and only the odd-chain propionyl fragment is glucogenic. Second, "beta-oxidation needs carnitine" — only for long-chain fatty acids; medium and short chains and their synthetic inhibitors enter independently, which is exactly why MCAD defects present with hypoketotic rather than ketotic hypoglycaemia. A third nuance: brown adipose tissue in neonates oxidises fat uncoupled through UCP1 (thermogenesis), an Indian viva favourite because newborn hypothermia programmes quote it.
Frequently asked questions
Which enzyme is the principal regulated step of adipose lipolysis?
Hormone-sensitive lipase, activated by catecholamines through cAMP and protein kinase A phosphorylation, suppressed by insulin.
How do free fatty acids travel in plasma?
Bound reversibly to albumin, which carries multiple fatty acid molecules per protein; only the tiny unbound fraction is metabolically active.
Why does malonyl-CoA block fatty acid oxidation?
It inhibits carnitine palmitoyltransferase-1, preventing long-chain acyl groups from entering mitochondria whenever synthesis is active in the fed state.
Can fatty acid carbon become glucose?
Not from even-chain fatty acids in humans, since acetyl-CoA cannot net-convert to oxaloacetate; glycerol backbones and odd-chain propionyl-CoA are glucogenic.
What is the ATP yield of complete palmitate oxidation?
Seven spirals yield 8 acetyl-CoA plus reduced cofactors, totalling about 106 ATP by current bioenergetic estimates.